Viability of ESUs Containing Multiple Types of Populations

This is the Independent Scientific Advisory Board's report and report submittal letter answering five questions asked by NOAA-Fisheries Northwest Science Center on January 25, 2005, about evaluating the viability of Evolutionarily Significant Units that contain hatchery and natural or resident and anadromous populations.

To answer these questions, the ISAB first defined a "Viable ESU" as a group of populations existing together as a metapopulation that as an entity is self-sustaining for the foreseeable future. A Viable ESU needs to be in an ecologically and evolutionarily functional state sufficient to: rebound from periods of low abundance or even very local extirpations that are expected natural phases of population cycles; avoid the loss of genetic and/or life history diversity during short-term losses in abundance; fulfill key ecological functions that are attributable to the species, such as nutrient cycling and food web roles; and provide for long-term evolutionary adaptability to changing environmental conditions.

Evaluation of ESU viability should rest not only on the numbers of component populations or on the abundance and productivity of those individual populations, but also should be based on the population dynamics within the ecosystem as a whole. This concept of ESU viability does not tolerate the loss of either the anadromous or resident life history form from any given ESU, because that loss would represent a loss in diversity for the ESU that would put its long-term viability at risk. In addition, although this concept of ESU viability may not preclude the presence of hatchery-origin individuals within an ESU, it does preclude the dependence of ESU viability on hatchery-origin individuals, and it precludes the replacement of the original wild population with a hatchery derived one. This argument is based on scientific evidence that an ESU needs to contain viable, naturally adapted populations inhabiting a variety of different natural habitats, interconnected as a metapopulation, if that ESU is to fulfill the entire complement of ecological and evolutionary interactions and functions.

Establishing the policy boundaries for ESU viability assessment is likely to be as important to the eventual outcome as the method used in the assessment. The natural populations associated with integrated hatchery programs are generally not themselves viable and the habitats upon which they depend are usually inadequate. If the policy decision is made that self-sustaining natural populations are not required for an ESU to be viable, it is likely that the number of extant natural populations will continue to decrease and the impetus behind current efforts to improve habitat conditions will be greatly reduced. The ISAB believes that the current science indicates that ESUs dependent upon hatchery production cannot be viable ESUs according to the ISAB's definition of this term uses in this report. Therefore, a policy that recognizes such ESUs as viable would need to use a definition of viability much different from the one the ISAB is using. The biological validity of such a definition would be questionable.

Complete ISAB findings and the answers to the NOAA questions are in the report.